A typical wastewater treatment plant (WWTP) produces one or more sewage sludges such as primary sludge and waste activated sludge. Some or all of the sewage sludge may be thickened up to about 6 wt % dried solids. The sewage sludge can be further treated in a mesophilic anaerobic digester. The resulting digestate is a Class B biosolid with no value or limited value.
Thermal hydrolysis is most commonly used in commercial anaerobic digesters according to the CAMBITHP process. In the typical form of this process, waste sludge is hydrolysed by a combination of heat (about 160 degrees C.) and pressure (at least several atmospheres) prior to anaerobic digestion. Because the viscosity of the waste sludge is reduced in the process, the waste sludge may be thickened, for example to about 8-16% dried solids by weight (DS) before hydrolysis. The anaerobic digester then operates at about 4-6% DS in the digester compared to 2-3% DS for a conventional digester coupled to a WWTP.
International Publication Numbers WO 2014/137218 and WO 2016/066752 give examples of the use of thermal hydrolysis devices in combination with anaerobic digesters.